The present invention relates to a foam structure having a surface with altered electrical attributes and a method of manufacturing same.
In applications where complex electronic circuits are used, the electronic noise generated by such circuits, i.e., the radio frequency/electromagnetic interference, may be of such level as to be hazardous or detrimental to nearby personnel or other electronic circuits. If such electronic noise reaches a high level, a unit may be in violation of federal regulations or the manufacturer's design specifications, requiring the unit to be recalled by the manufacturer. Applications where this is important include low observable or low radar bounce back uses that require radar absorbing materials or structures. Some of these applications include frequency selective services, anechoic chambers, antenna termination, electric and magnetic shielding, and thermal-electric shielding.
One solution to the above problem involves redesigning the circuit components to reduce the radio frequency/electromagnetic interference to acceptable levels. However, this is a costly remedy. It has also been proposed that a foil shield be placed around the electronic circuitry and connected to ground as another viable approach to reducing environmental radiation from electronic circuitry. Toward this end, an aluminum or copper foil has been adhesively coated on both sides of a substrate, and an outer wrapping layer of polyester or plastics material has been applied thereto. However, these metal foil shielding arrangements are relatively costly and are not especially durable.
Electromagnetic interference has been reduced within a microwave module by producing a sheet of absorber material, such as an elastomer material filled with iron powder, cutting out a pattern from the sheet of absorber material to fit around the components within the module (this pattern generally being relatively complex in shape), and then bonding the absorber to the module lid with an adhesive. Foam has been used as the module lid, and for other structures requiring absorber material. However, the precision shaping and positioning required of the sheet of absorber material significantly increases cost.
Another solution involves R-cards (i.e., resistive or conductive ink cards), examples of which are disclosed in U.S. Pat. Nos. 5,494,180 and 5,364,705, both of which are entitled "Hybrid Resistance Cards and Methods of Manufacturing Same," and are incorporated herein in their entireties. R-cards have been fabricated by screen printing an ink having electric altering properties imparted by resistive or conductive ingredients onto a major surface of a carrier. The ink printed carrier is then bonded to a part, such as a foam structure. By using silk screening to apply the inks onto a card, the resolution of the patterns as fine as 1 mil permits the R-card to have an accurate and precise resistivity curve.
However, the R-card carriers have been flat, and may be relatively stiff, cards of various materials. For example, one approach has been to print the ink on a cured epoxy-glass laminate and then subsequently use separate film adhesive material to bond such an R-card to a part. Such an approach increased costs and labor, e.g., due to the adhesive coat operation. Also, R-card rigidness has the drawback of not readily conforming to non-flat surfaces, such as double curved surfaces.
R-film has been developed to overcome some of the difficulties in using R-cards with structures of complex geometries while maintaining the highly accurate and precise resistivity curve. R-film is disclosed in co-pending U.S. Patent Application entitled "Screen Ink Printed Film Carrier and Methods of Making and Using Same from Electrical Field Modulation" filed Dec. 10, 1997, now U.S. Pat. No. 5,890,429 which is incorporated herein in its entirety. R-film is a silk screen printed, electrical altering image on thin adhesive film carrier that is flexible to conform to surfaces that are flat and non-flat. However, the process for using R-film requires multiple steps for printing the film and adhering the film to the structure.
It is known to directly coat surfaces of foam structures using techniques that do not provide high resolution. Some of these techniques are spraying, dipping, and loading the foam with resistive and/or conductive filters. The problem with not having high resolution printing is that the accuracy and precision of predetermined resistivity tapers are compromised.
What is needed, therefore, is a foam structure, and manufacturing method thereof, with a surface having a predetermined resistivity taper of relatively high accuracy and precision that is relatively simple to manufacture.